Sealed contact relays are well known in the electrical arts and have long found extensive application in electrical systems for performing a wide range of switching functions. A typical dry reed form of such relays comprises a pair of overlapping reed springs of a magnetically responsive and electrically conductive material suspended at their ends by an envelope, usually glass, in which they are sealed. A winding encircling the envelope is energized to generate a magnetic field for actuating the reed springs to control the electrical circuit in which the relay is connected. Such dry reed relays have served well in particular circuit applications; the character of their contact surfaces, however, renders them essentially current limited. Minute imperfections on the contact surfaces reduce the areas of electrical contact to less than the entire surface of the contact. As a result, currents of a magnitude beyond a predetermined limit tend to cause melting at the contact areas, which, in turn, increases the tendency of the contacts to stick closed after actuation. To counteract this tendency, greater reed spring retractile forces are required which increase the amount of magnetic flux and, therefore, coil current required.
When larger current carrying capacities are required, this problem has been largely overcome by the employment of the well-known mercury-wetted relay. Typically, the construction of such a relay comprises a contact-mounting pole-piece suspended at one end of a glass envelope and a substantially coaxial stem suspended at the other end, each of the elements extending partially into the envelope from its ends. Connected to the stem by means of a flexible hinge is an armature dimensioned to overlap in a spaced-apart relation with the pole-piece contact. The surfaces of the opposing contact and armature face are coated with a film of mercury. When these surfaces meet upon energization of the relay, electrical connection is uniformly established over the entire contact surfaces. As a result, the magnitude of the currents carried by the relay required to cause contact surface melting is significantly increased. As a further and important result, the current values necessary to actuate the relay are substantially reduced. Since the force required to separate the contacts need not contend with contact melting, the stiffness of the armature or, more precisely, its hinge, may be substantially less than that of the dry reed relay counterpart. The lower limit of armature hinge stiffness of a mercury-wetted relay is determined only by the requirement that the armature restoring force be sufficient to overcome the mercury surface and viscous forces and hence rupture the mercury bridge between the contacts. This lower armature hinge stiffness therefore allows mercury-wetted switches to require lesser magnitudes of magnetic flux for their operation than their dry reed counterparts while having substantially improved load switching capabilities. Although broadly similar in structure and function to their dry reed counterparts, practical aspects of the fabrication and operation of mercury-wetted relays have resulted in structural arrangements and problems peculiar to such relays.
One such problem, for example, results from the greater flexibility of the armature member hinge which increases the tendency for oscillation of the member after its release. Such oscillation ordinarily presents little difficulty in dry reed relays as long as the first (and greatest) return swing falls short of re-establishing electrical contact. In the case of mercury-wetted relays, release of the electrical contacts requires the rupturing of a mercury bridge at the contact area as mentioned hereinbefore. This initially leaves a "pillow" of mercury at the pole-piece contact with which electrical contact may be re-established at the first return swing of the contacting member in a release oscillation, thereby producing undesirable contact bounce. As a result, prior art mercury-wetted relay constructions provide some means for dampening such oscillations. In one well-known arrangement, for example, the armature contacting member is positioned with respect to its enclosing glass envelope so that contact with the inside of the latter envelope is made to shorten the first and succeeding swings to dampen the oscillation. However, a disadvantage of this method is that precise control of the spacing between the armature and the glass is difficult to achieve in manufacture.
Another consideration in the construction and operation of a mercury-wetted switch is the achievement of proper response to the application of a given operating magnetic field. Both the armature assembly stiffness and the reluctance of the flux closure path through the relay elements are important factors in determining this relay sensitivity. In many prior art constructions, obtaining tight control over the armature assembly stiffness has been a problem in manufacture because the location of the points at which the hinge is affixed to the stem and to the armature must be precisely controlled and the hinge must not come in contact with either the armature or the stem in the region between these points of attachment when the switch is operated. Otherwise the stiffness, and hence the magnetic flux required for operation, will be increased. One critical point in the flux closure path governing its reluctance is the hinge juncture between the armature member and the face of the stem. Because of the thin cross-section required to obtain low stiffness, the hinge can carry only part of the magnetic flux required to operate the switch, the majority being shunted across the gap between the armature and the stem face.
Another factor influencing the operate sensitivity of miniature mercury switches relates to the proper drainage of mercury back into the reservoir following shipment or handling. When mercury is transferred from the reservoir to the space between the armature member and the envelope, surface tension forces pull the armature toward the envelope unless mechanically restrained, thereby increasing the magnetic gap and the magnetic flux required for operation. In severe cases the switch may even fail to operate. Accordingly, objectives in the design of the switch for mercury-wetted relays include: minimization of the armature stem-gap reluctance; an armature assembly design which facilitates the control of stiffness in manufacture; an easily manufacturable method for minimizing the armature oscillation following release; and minimization of mercury drainage problems which could adversely affect the operate sensitivity of the relay. It is to the foregoing and other problems in the fabrication of mercury-wetted relays that the construction of this invention is directed.